The present invention relates to piezoelectric devices and more particularly to a piezoelectric transformer that utilizes two input drive sections.
PZT transformers are basically energy converters. A PZT transformer converts an electrical input into mechanical energy and subsequently reconverts this mechanical energy back to an electrical output. The mechanical transport causes the PZT transformer to vibrate, similar to quartz-crystal operation, but at acoustic frequencies. The resonance associated with this acoustic activity is extraordinarily high; Q factors greater than 1000 are typical. This transformer action results from using properties of certain ceramic materials and structures. The physical configuration and number of layers in its construction set a PZT transformer""s voltage gain.
Many materials, such as quartz, lithium niobate, and lead-zirconate-lead-titanate (PZT) exhibit some form of the piezoelectric effect. The piezoelectric transformer uses PZT, hence, it is a PZT transformer. Two piezoelectric effects exist: the direct effect and the inverse effect. With the direct effect, placing a force or vibration (stress) on the piezoelectric element generates a charge. The polarity of this charge depends on the orientation of the stress compared with the direction of polarization in the piezoelectric element. During the manufacturing process, poling, or applying a high dc field in the range of 45 kV/cm to the PZT transformer, sets the polarization direction.
The inverse piezoelectric effect is, as the name implies, the opposite of the direct effect. Applying an electric field, or voltage, to the piezoelectric element results in a dimensional change, or strain. The direction of the change is likewise linked to the polarization direction. Applying a field at the same polarity of the element results in a dimensional increase, and fields of opposite polarity result in a decrease. An increase in one dimension in a structure results in a decrease in the other two through Poisson""s coupling, or the fact that lateral strain results in longitudinal strain at Poisson""s ratio. This phenomenon is an important factor in the operation of the transformer.
The piezoelectric transformer uses both the direct and inverse effects to create high-voltage step-up ratios. A sine-wave voltage drives the input portion of the transformer, which causes it to vibrate. This operation is the inverse, or motor, effect. The vibration couples through the structure to the output to generate an output voltage, which is the direct, or generator, effect.
The piezoelectric transformer is constructed of PZT ceramic, but more precisely it is a multilayer ceramic. The manufacturing of the transformer is similar to the manufacturing of ceramic chip capacitors. The process prints layers of flexible, unfired PZT-ceramic tape with metallic patterns, then aligns and stacks the layers to form the required structure. The next step involves pressing, dicing, and firing the stacks to create the final ceramic device.
The input section of the transformer has a multilayer ceramic-capacitor structure. The pattern of the metal electrodes creates an interdigitated plate configuration. The output section of the transformer has no electrode plates between the ceramic layers, so firing produces a single ceramic output structure. Conductive material, which forms the output electrode for the transformer, coats the end of the output section.
The next construction step establishes the polarization directions for the two halves of the transformer. Poling of the input section across the interdigitated electrodes results in a polarization direction that aligns vertically to the thickness. Poling of the output section creates a horizontal or length-oriented polarization direction. Operating the transformer drives the input in thickness mode, which means that an applied voltage between the parallel plates of the input causes the input section to become thicker and thinner on alternate halves of the sine wave. The change in input thickness couples through to the output section, causing it to lengthen and shorten and thereby generating the output voltage. The resulting voltage step-up ratio is proportional to the ratio of the output length and the thickness of the input layers.
Past piezoelectric transformers are based on the well-known Rosen design (U.S. Pat. No. 2,830,274). These high voltage transformer designs are of a piezoelectric ceramic plate which includes a single driving section and a driven section which each have different polarizations. The different polarizations provide for the voltage transformation in these designs. However, these designs have several drawbacks. In particular, the output voltage from the driven section is controlled either by changing the voltage amplitude applied to the driving section or moving the input voltage frequency off the resonant frequency of the transformer. The first method requires a drive regulator, such as a buck regulator, which introduces losses and lowers efficiency. The second method is difficult to control due to the high Q of the transformer and also results in a loss of efficiency. Besides controlling the output voltage by the amplitude and frequency methods mentioned above, duty cycle and phase methods are also available.
Adjustable gain is a common requirement in several applications. For example, in LCD backlights for laptop computers, a constant battery voltage is provided (usually 10 to 20 volts) and the driving transformer is required to have adjustable gain in order to provide adjustable screen brightness.
Prior methods having attempted to provide adjustable again so as to make this parameter independent of the other design parameters. Two of these prior art methods for providing adjustable gain have included frequency modulation and pulse width modulation. Frequency modulation provides adjustable gain as a function of driving the transformer at frequencies that are off resonance. The further off resonance the transformer is driven the less output amplitude is produced and the less gain it has. The off resonance condition has the disadvantage of operating the transformer at less than optimum efficiencies because the piezoelectric transformer is not being driven at a resonance point. Moreover, because the transformer is a high Q device, the resonant frequency peak is very narrow and the slope is very steep making it difficult to control the working point on the slope or keep it on the same side of the slope, and therefore the gain is adversely affected. In addition, the transformer frequency will drift as the operating temperature changes.
A driver circuit in frequency modulation uses an error signal between the desired output and the actual output to change the frequency. The change in frequency required depends on a slope of the gain versus frequency curve. However, this slope varies both in magnitude and polarity which makes a feedback scheme difficult to control. Stability and convergence can only be maintained if the slope polarity is constrained. Moreover, the high Q nature of the transformer and frequency variations with temperature and loading further complicate the operation of such a driver circuit.
Pulse width modulation provides adjustable output voltage as a function of the duty cycle of the driving signal. Changing the duty cycle of the driving signal from a nominal 50% duty cycle lowers the amplitude of the fundamental frequency which reduces the output voltage at that frequency. Pulse width modulation has the disadvantage of diverting power to harmonic frequencies which reduces efficiency and introduces unwanted signals, also.
For example, U.S. Pat. No. 5,747,914 to Huang et al. entitled xe2x80x9cDRIVING CIRCUIT FOR MULTISECTIONAL PIEZOELECTRIC TRANSFORMERS USING PULSE-POSITION-MODULATION/PHASE MODULATIONxe2x80x9d issued on May 5, 1998 and assigned to the assignee of the present invention, discloses a Rosen transformer that is phase driven at opposite ends of the transformer.
The present invention provides at least two input driving sections for a piezoelectric transformer. The two input driving sections are adjacent, preferably stacked along the thickness dimension of the transformer, with either one section on top of the other or the sections interlaced. Preferably, each section is dimensionally equivalent so that the respective displacements have the desired combinational effect. The two input driving sections provide for the application of phase-modulated input signals to control the gain or amplitude of the transformer output.
One embodiment of the present invention uses four electrode patterns that are grouped into two pairs at one longitudinal (or length) end of the transformer. The first pair is stacked above the second pair. Connections to the electrodes can be provided by connections on the sides of the transformer or by vias. Alternatively, the pairs can be interlaced so that the electrode stack contains alternating pairs of electrodes. A further embodiment provides at least two input driving sections at both ends of the transformer. The output section for this embodiment preferably is disposed between the ends.
Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiment thereof, from the claims and from the accompanying drawings in which details of the invention are fully and completely disclosed as a part of this specification.